Thunderbird gets Twitter support via TweeQuilla proof-of-concept add-on
The new add-on is called TweeQuilla, and it's just at version 0.1. TweeQuilla shows the tweets as messages in the message pane, and if a tweet includes a URL, it uses the preview pane to show that URL.
However, the developer is looking for cohorts to help him bring the project to Linux and OS X as well. Still, if you're a Windows Thunderbird user, this might help push your aging 'bird into the realm of social networking.